The present invention relates to a method for activating fuel cells, particularly direct oxidation fuel cells, which directly utilize organic fuel.
Fuel cells are classified into phosphoric acid type, alkaline type, molten carbonate type, solid oxide type, solid polymer type, etc., according to the kind of the electrolyte they use. Among them, solid polymer fuel cells, which are characterized by low-temperature operation and high output density, are becoming commercially practical in such applications as automobile power sources and domestic cogeneration systems.
Meanwhile, the functions of portable devices, such as notebook personal computers, cellular phones, and personal digital assistants, are becoming increasingly more sophisticated, and the electric power consumed by such devices tend to increase commensurately. Under such circumstances, it is feared that improvements in energy density of lithium ion secondary batteries and nickel-metal hydride secondary batteries, which are currently the power sources for portable devices, are unable to keep up with the increase in power consumption, thereby shortly causing a problem of capacity shortage of such power sources.
As the power source to solve this problem, solid polymer fuel cells (hereinafter referred to as “PEFCs”) have been receiving attention. Among them, most expected are direct oxidation fuel cells, which can generate electric energy by directly oxidizing a liquid fuel at ordinary temperature at the electrode without the need to reform it to hydrogen, because they require no reformer and can be readily miniaturized.
Low-molecular-weight alcohols and ethers have been examined as the fuel for direct oxidation fuel cells. Among them, most promising is methanol, which offers high energy density and high output. Fuel cells that use methanol as the fuel are called direct methanol fuel cells (hereinafter “DMFCs”).
The anode reaction and the cathode reaction of a DMFC are represented by the following equations (1) and (2), respectively. Oxygen serving as the oxidant at the cathode is commonly taken from air.CH3OH+H2O→CO2+6H++6e−  (1)3/2O2+6H++6e−→3H2O  (2)
The ion-conductive electrolyte membrane, which transfers protons produced at the anode to the cathode, is often a perfluorosulfonic acid membrane, such as Nafion (registered trademark), in the same manner as in PEFCs utilizing hydrogen as the fuel. Also, such an electrolyte membrane usually has, on each side, a catalyst layer including a catalytic substance. This catalyst layer is generally prepared, with the aim of ensuring proton conductivity, by applying a mixture of a catalytic substance and a solution containing a perfluorosulfonic acid, which is the same component as that of the electrolyte membrane, and drying the mixture.
Solid polymer type electrolyte membranes that are currently used exhibit sufficient ionic conductivity only when they are hydrated. It is thus necessary to hydrate the electrolyte membrane in a process of fabricating a fuel cell. However, a polymer electrolyte membrane absorbs or desorbs water relatively promptly, depending on the ambient temperature and humidity environment, and undergoes dimensional changes accordingly. Hence, it is extremely difficult to fabricate a fuel cell, with its polymer electrolyte membrane hydrated.
For this reason, a fuel cell is often subjected to a hydration treatment, called an activation treatment, after it has been fabricated, with its electrolyte membrane being in a dry state, to the extent that dimensional changes of the electrolyte membrane would cause no special problem.
When a fuel cell is left unused for a long period of time, with the gas communication with the outer environment blocked off, or without any humidification measures such as the supply of moisture to the electrolyte membrane, the proton conductivity of the electrolyte membrane lowers remarkably. In this case, the manipulation of re-supplying moisture to the electrolyte membrane, i.e., activation may become necessary as well.
Such an activation method is described in Japanese Laid-Open Patent Publication No. Hei 6-196187. According to this proposal, while humidified hydrogen gas and humidified oxygen gas are supplied to the anode and the cathode, respectively, a voltage of 1.3 V or more is applied between the two electrodes. The resultant electrolysis of water in the electrolyte membrane produces hydrogen and oxygen to cause a large increase in the concentration gradient of water, thereby increasing the diffusion speed of water. As a result, the electrolyte membrane is promptly hydrated according to this method.
Also, another method is proposed in U.S. Pat. No. 6,596,422. First, humidified hydrogen gas and air are supplied to the anode and the cathode, respectively, for power generation, and then, methanol fuel and air are supplied to the anode and the cathode, respectively, for operation.
As described above, various proposals have been made to hydrate the electrolyte membrane, but to perform an operation requiring humidified hydrogen, hydrogen supply equipment and hydrogen-humidifying equipment are necessary. Generally, hydrogen gas is highly flammable and has a wide explosion limit concentration range, so that it must be handled with great attention. Therefore, there is a need to incorporate sufficient safeguard equipment into a manufacturing process that may possibly use large amounts of hydrogen.
The manufacturing process of PEFCs using hydrogen as the fuel inherently has such equipment. However, in the manufacturing process of DMFCs, which are expected to be mass-produced as the power sources for portable devices, the manufacturing facilities need to be simplified as much as possible, and hence such equipment requiring hydrogen is undesirable.
The method of electrolyzing water in an electrolyte membrane requires application of a voltage of 1.3 V or more per unit cell. Thus, a problem of this method is that if the number of cells of a fuel cell stack is increased, the voltage applied to the fuel cell stack becomes high.
Further, the present inventors believe that the following problems should also be resolved.
The catalyst used in direct oxidation fuel cells is typically platinum, a compound thereof, or a mixture thereof. While platinum is a substance that is inherently resistant to oxidation, it is known that its surface is easily oxidized if baked at high temperatures in an atmosphere containing oxygen. It is therefore considered that such oxidation of platinum decreases the number of effective active sites for reaction.
Such oxidation is unlikely to occur in the manufacturing process of a catalyst for fuel cells, since the catalyst is baked at high temperatures in an inert atmosphere or a reducing atmosphere, but the decrease in the number of active sites due to the oxidation of platinum surface occurs in the manufacturing process of fuel cells, for example, for the following reason.
As described above, a catalyst layer is composed of a catalytic substance and an electrolyte substance, but a mere application of a mixture of these two substances does not facilitate the proton transfer at the interface between the electrolytes in the catalyst layer or the interface between the electrolyte in the catalyst layer and the electrolyte membrane. To improve such proton transfer, it is common to perform a hot pressing process in which the mixture is heated to a temperature equal to or higher than the glass transition point of the electrolyte and pressed, in order to enhance the joint between the electrolytes. While such a process is usually performed for a short period of time of several minutes, it may be performed for a relatively long period of time at high temperatures. This process can possibly promote the oxidation of platinum surface by the oxygen in air.
Further, platinum surface can be oxidized not only in the fabrication process of a fuel cell but also during the use or storage of a fuel cell, because a cathode catalyst in particular is exposed to a relatively high potential in the presence of oxygen for an extended period of time, or hydrogen peroxide, which is a strong oxidant, is produced as an intermediate product of a side reaction at the cathode.
Regarding the above-mentioned problems, there have been no activation methods that perform a regeneration treatment specifically and effectively.
It is therefore an object of the present invention to solve the above-mentioned problems, enable initial activation of a fuel cell upon its fabrication and reactivation after a long-time suspension of its operation, and provide a fuel cell with sufficient activity at low costs.